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Walk down the magnesium aisle and you will find oxide, citrate, glycinate, malate, chloride, taurate, orotate and L-threonate. Some cost a few dollars. Some cost thirty. All of them say magnesium on the front.
They are not marketing variations on one product. They are genuinely different chemical compounds, they contain genuinely different amounts of magnesium per milligram, and the number printed largest on the bottle is frequently not the number you should be comparing.
This article explains what the forms are and how to read their labels against each other. If you are after the background — what magnesium is and which foods carry it — start with Magnesium 101. This piece picks up where that one stops: at the shelf.
Magnesium is never sold as magnesium
The first thing to understand is that you cannot buy magnesium. Elemental magnesium is a reactive metal; it is not something that goes into a capsule.
What goes into a capsule is a magnesium salt or chelate — magnesium bound to something else. Bound to oxygen it becomes magnesium oxide. Bound to citric acid, magnesium citrate. Bound to two molecules of the amino acid glycine, magnesium glycinate.
That partner molecule has weight. It takes up room in the capsule. And the heavier it is, the less of the capsule is magnesium.
This is why the same “500 mg” can mean wildly different things, and it is the single most useful thing to know before comparing two bottles.
The elemental percentages, and the arithmetic behind them
Every figure in this table is calculated directly from the compound’s molecular formula: the atomic weight of magnesium divided by the molecular weight of the whole compound. Nothing here is quoted from a manufacturer.
| Form | Approximate elemental magnesium |
|---|---|
| Magnesium oxide | 60% |
| Magnesium carbonate | 29% |
| Magnesium chloride (anhydrous) | 25% |
| Magnesium sulfate (anhydrous) | 20% |
| Magnesium citrate | 16% |
| Magnesium malate | 16% |
| Magnesium glycinate (bisglycinate) | 14% |
| Magnesium lactate | 12% |
| Magnesium chloride (hexahydrate) | 12% |
| Magnesium sulfate (heptahydrate, “Epsom salts”) | 10% |
| Magnesium taurate | 9% |
| Magnesium aspartate | 8% |
| Magnesium L-threonate | 8% |
| Magnesium orotate | 7% |
Worked example, so you can check it: magnesium oxide is MgO. Magnesium’s atomic weight is 24.305 and oxygen’s is 15.999, giving a molecular weight of 40.304. Divide 24.305 by 40.304 and you get 0.603 — sixty per cent.
Two warnings about using this table.
Hydrated forms differ from anhydrous ones. Water bound into the crystal adds weight, which is why magnesium chloride appears twice above at 25% and 12%. Labels do not always specify which they used.
Branded complexes are not on this table. Several patented magnesium ingredients are mixtures or proprietary chelates, and their elemental content is whatever the manufacturer declares rather than something you can calculate. For those, the declared figure on the panel is the only figure.
What this means in practice
Take two bottles, both saying 500 mg.
A 500 mg magnesium oxide capsule contains roughly 300 mg of magnesium. A 500 mg magnesium glycinate capsule contains roughly 70 mg of magnesium.
That is more than a fourfold difference in the mineral itself, between two products whose fronts look identical.
Now the important qualification, because the obvious conclusion from those two numbers is the wrong one.
More magnesium in the capsule is not the same as more magnesium absorbed
The amount in the capsule and the amount that crosses the gut wall are two different quantities, and the ranking of the forms is not the same on both. Our guide to bioavailability covers the general principle; this is how it plays out for one mineral.
The National Institutes of Health’s Office of Dietary Supplements puts the general principle plainly: forms of magnesium that dissolve well in liquid are more completely absorbed in the gut than less soluble forms. Its fact sheet notes that magnesium in the aspartate, citrate, lactate and chloride forms is absorbed more completely and is more bioavailable than magnesium oxide and magnesium sulfate.
So the form with the highest elemental percentage on the table above is also among the least well absorbed. The cheap bottle is cheap for a reason, and the reason is real.
That does not automatically make the expensive bottle better value either. Absorption is a percentage of what is there, and a high percentage of a very small amount is still a very small amount. The two numbers have to be considered together, and the honest position is that the comparative absorption research is limited enough that nobody should be quoting precise multipliers at you.
What you can do is compare like with like: elemental milligrams per capsule, cost per capsule, and form. Three columns, and any two products become comparable.
The forms you will actually see
Oxide. The highest elemental content and the lowest cost. Poorly soluble, and one of the forms most commonly associated with a laxative effect.
Citrate. Bound to citric acid. Better solubility than oxide. Widely available and inexpensive, which makes it the usual default from a value standpoint.
Glycinate, or bisglycinate. Magnesium bound to two glycine molecules. Low elemental content by weight, which is why these capsules are often large or the serving size is two or three.
Malate. Bound to malic acid. Similar elemental content to citrate.
Chloride. Sold both orally and as topical “magnesium oil” and flakes. Note that claims about absorption through skin are a separate question from oral absorption, and the evidence base for the topical route is thin — treat topical products as a different product category rather than an alternative delivery of the same thing.
Sulfate. Epsom salts. Bath products, principally.
Taurate, orotate, L-threonate. The premium tier, with the lowest elemental percentages on the table and the highest prices per milligram of magnesium. Each is marketed on a specific proposition. The research on each is at an earlier and thinner stage than for the long-established forms, and the honest summary is that the marketing is considerably ahead of the evidence in this tier. Read the elemental figure on these especially carefully — at 7 to 9 per cent, the capsule is mostly the partner molecule.
“Magnesium complex” or blends. Several forms in one product. Check whether the panel breaks out each form’s contribution or gives only a combined total; if it is a proprietary blend, it does not have to tell you the split.
The two reference numbers, and why one is smaller than the other
This trips up almost everyone, so it is worth setting out carefully.
The RDA for magnesium for adults runs from 310 to 420 mg per day depending on age and sex. That figure covers magnesium from all sources — food first, and food is where most people get most of theirs.
The upper limit for supplemental magnesium is 350 mg per day for ages nine and over.
So the upper limit is lower than the recommended intake, which looks like a contradiction and is not. They are measuring different things. The RDA counts everything you eat and swallow; the upper limit counts only what comes from supplements and medications, and explicitly excludes magnesium from food. The reason for the separation is that magnesium arriving in a concentrated dose behaves differently in the gut from magnesium spread through a day’s meals.
Where this matters on a label: a single capsule can exceed the supplemental upper limit on its own, and nothing on the packaging is required to point that out to you.
What high supplemental doses commonly do
Gastrointestinal effects are the common and well-documented consequence of too much supplemental magnesium: diarrhoea, nausea and abdominal cramping. The NIH fact sheet identifies magnesium carbonate, chloride, gluconate and oxide as the forms that most commonly produce diarrhoea.
This is worth knowing as a label-reading matter, because it is the most likely reason a cheap high-elemental product turns out not to suit somebody.
Interactions — the section to actually read
Magnesium supplements interact with several widely prescribed medicines. This is not an exhaustive list, and it is a conversation to have with a pharmacist rather than a checklist to self-manage, but these are the documented ones:
- Bisphosphonates — absorption of the medicine is reduced; doses are generally separated by at least two hours.
- Tetracycline and quinolone antibiotics — magnesium binds them in the gut; separation of two to six hours is the usual guidance.
- Loop and thiazide diuretics — these increase urinary magnesium loss.
- Proton pump inhibitors — long-term use has been associated with low blood magnesium.
If you take prescription medication of any kind, take the bottle to your pharmacist before you take the capsule. It is a thirty-second conversation and they will do it happily.
What the label will not tell you
Four things, none of which appear on any panel.
Whether this form suits you. Individual response to supplemental magnesium varies, particularly in the gut, and finding that out is a matter of your own experience and your doctor’s advice.
Whether you need it at all. That is a clinical question.
What the absorption actually was. Declared content is a manufacturing fact; absorbed quantity is an individual outcome, and no label can promise the second.
Whether the contents match the declaration. That is what third-party verification programmes exist to check.
The buying checklist
- Find the elemental magnesium figure in the nutrient table, not the compound weight on the front.
- Note the serving size — one capsule, or three?
- Work out elemental milligrams per capsule.
- Work out cost per 100 mg of elemental magnesium, which is the only genuinely comparable price.
- Check the form against the table above, and check whether it is hydrated.
- Read the other ingredients list for anything you avoid.
- Check your medicines with a pharmacist before you buy.
Sources
- National Institutes of Health, Office of Dietary Supplements, Magnesium — Fact Sheet for Health Professionals.
- U.S. Food and Drug Administration, Dietary Supplement Labeling Guide: Chapter IV, Nutrition Labeling.
- Elemental percentages calculated from standard molecular formulas and IUPAC atomic weights, not sourced from any manufacturer. The worked example above lets you verify the method.
This article is a guide to reading packaging and comparing products. It is general information only and is not medical advice. It makes no claim that magnesium supplementation benefits any condition, it recommends no dose, and it does not diagnose, treat, or cure any condition. Speak with a qualified healthcare professional before starting any supplement.